The techniques and systems disclosed herein relate to the field of optical spectroscopy. More specifically, highly efficient means by which moderate resolution spectroscopy may be performed in or below the region of the electromagnetic spectrum referred to as the vacuum ultra-violet (VUV).
Optical spectroscopy techniques have been employed in the characterization of matter for well over a century. While some of the earliest spectroscopic tools employed dispersive prisms to spatially separate wavelengths of light, the vast majority of modern instruments utilize diffraction elements for this purpose. Grating-based systems are generally capable of much higher resolving power and their utilization has become wide spread as a result of significant developments in the production and subsequent replication processes used to manufacture high quality grating elements.
The majority of commonly utilized optical spectroscopic techniques are performed using wavelengths either above the VUV (in the deep ultra-violet, visible or infrared) or below it (in the X-ray region). Of those techniques that employ VUV wavelengths, virtually all of them involve high resolution instrumentation. As compact high brightness sources are not generally available in this energy space, many of these systems are used in conjunction with massive synchrotron radiation sources at national laboratories.
Designing high efficiency spectroscopic instruments for operation in the VUV has proven to be a formidable challenge. Standard reflection gratings are furnished with an evaporated Al coating to enhance their reflectivity. This approach works well over a wide range of wavelengths extending from the deep ultra-violet (DUV) to the near-infrared (NIR). For operation in the VUV, however, it is necessary to protect the aluminum films with a MgF2 overcoat (˜250 Å) to prevent oxidation, which can drastically reduce the reflectivity of aluminum at wavelengths lower than 170 nm.
Even with the addition of protective coatings, the normal incidence reflectivity of Al (and most other metals) decreases significantly in the VUV. Consequently, VUV monochromator, designs have traditionally incorporated concave gratings in order to eliminate reflective surfaces for efficiency reasons. Notable examples of such single element instruments include devices based upon the Rowland circle and Seya-Namoika mounts (see Masato Koike, “Normal-Incidence Monochromators and Spectrometers”, in Vacuum Ultraviolet Spectroscopy (J. A. R. Samson and D. L. Ederer, ed.), Vol. II, pp. 1-20. Academic Press, San Diego, 2000). A common drawback of these simple designs is the presence of astigmatism which results in a loss of intensity and spatial resolution in the direction parallel to the entrance slit. More importantly, while finely ruled gratings can achieve high spectral resolution their VUV efficiency profiles are generally quite low and routinely exhibit complicated wavelength dependencies.
To overcome the diminished normal incidence reflectivity of metals in the VUV, spectroscopy systems based on grazing incidence grating mounts have been employed in some circumstances. Unfortunately, such systems typically are designed for use in large scale, high-resolution, beam-line experiments and as such, incorporate optical elements (gratings and mirrors) ill-suited for incorporation into small footprint commercial instruments. For example, such systems may have a very large angle of incidence relative to the grating normal. The angle of incidence and very long focal lengths (on the order of 100 cm) do not lend themselves to integration in small footprint systems.
In situations where resolution requirements are modest there would be benefit in designing a compact VUV spectroscopic instrument which overcomes the abovementioned shortcomings by employing an optical element that separates, spreads or disperses light into spatially separate wavelengths in a compact spectrometer system.
A number of prism-based VUV monochromators have been designed specifically for scientific research applications. See for example H. W. Moos et. al., Appl. Opt. 9, 601 (1970) and P. G. Moyssides, et. al., J. Mod. Opt. 47, 1693 (2000). These instruments employ dispersive prisms mounted on rotation stages, along with exit slits and single element detectors to provide wavelength scanning capabilities.
A select number of prism-based instruments have been designed to operate in conjunction with multi-element array detectors, so as to enable the simultaneous collection of multiple wavelengths. One such instrument, built for the Naval Research Laboratory, is described by L. Rickard, et al. in Proceedings of SPIE 1937, 173 (1993). A second such instrument, built for NASA, is described by J. T. Rayner et al., in Publications of the Astronomical Society of the Pacific 115, 262 (2003). Warren et al. in U.S. Pat. No. 5,127,728 also discloses a prism spectrometer designed for use in combination with multi-element array detectors. Also in the prior art, Wang et al. in U.S. Pat. No. 6,744,505 discloses an imaging spectrometer for use in general spectroscopic applications where the wavelength dispersive element is a prism.